Perhaps a plot where the she - monsters have a special power that only Percy can unlock or control. For instance, they hold the key to a powerful weapon, and Percy has to earn their trust to access it. This would involve Percy going through a series of trials set by the she - monsters.
One plot idea could be that the she - monsters are actually the guardians of an ancient prophecy related to Percy. So, Percy has to work with them to decipher the prophecy and prevent a catastrophic event from happening. This would involve a lot of exploration into their culture and mythology to understand the prophecy better.
One idea could be Percy having to face a new type of monster that is a combination of all the previous ones he's fought. This super - monster could be created by an ancient and powerful enemy who wants to truly become the king of monsters. Another plot could involve Percy having to enter the realm of the dead to find a weapon that can defeat the king of monsters. Maybe he has to make a deal with Hades to get this weapon.
One idea could be Percy having to face a new type of monster that is a hybrid of multiple known ones. This would test his skills in new ways.
One idea could be exploring the origin of the Primordial of Monsters in more depth. Maybe there's a hidden prophecy related to it that Percy and his friends have to uncover. Another could be a story where the Primordial of Monsters forms an unexpected alliance with Percy against an even greater threat.
Well, a great plot for this fanfiction could be that Percy discovers a new type of monster that has a connection to the huntresses' past. Maybe it was a monster that was once thought to be extinct but has now returned. The huntresses and Percy then have to research the old tales about this monster to find its weakness. As they dig deeper, they also uncover secrets about the huntresses' order that change their perspective on their own history and mission.
One idea could be Percy having to protect his harem from a new and powerful monster threat that specifically targets them because of their connection to him. Another could be Percy exploring his relationships within the harem while also dealing with his identity as the son of Poseidon in a world full of monsters.
One unique element is Percy's character development. As he tames monsters, he has to learn about their nature and needs, which makes him more empathetic and wiser.
A cool plot could involve Percy as a super soldier being sent on a secret mission by the gods. This mission might be related to a new threat that combines the mortal world's technology and the mythological realm. He would have to use his super soldier skills, like enhanced strength and combat training, along with his demigod powers to succeed.
One unique plot idea could be that Percy discovers he has a special connection to the dragons because of his demigod powers. Maybe he can communicate with them on a deeper level than others. Another idea is that the dragons are the key to solving a new threat to the demigod world, like a powerful ancient evil that only the combined strength of the demigods and the dragons can defeat.
Well, perhaps Percy is raised by a group of monsters who are actually being controlled by an even more powerful and malevolent force. Percy then has to figure out how to break the control over the monsters that raised him and turn them into allies. He could start having strange dreams or visions that give him clues about his true heritage and his purpose in this situation. And as he grows up, he starts to notice differences between himself and the monsters, which makes him question everything he's been taught.
One idea could be a story where Percy, the main character, discovers a new type of monster that has unique powers. He has to figure out how to communicate with it and whether it's a friend or foe. Another idea is a fanfic set in a different time period, like Percy in a medieval world full of monsters, and he has to use his skills in a whole new way. Maybe he becomes a knight who battles the most dangerous monsters.